1. Field of the Invention
The present invention relates to a bi-directional horizontal readout register of a CCD imaging sensor. In particular, the invention relates to use of a non-destructive readout floating gate sensing electrode to sense the readout register and a vertical anti-blooming structure to reset the horizontal readout register.
2. Description Of Related Art
Charge coupled devices (hereinafter CCDs) are widely used in video imaging and recording applications. For example, the architecture of a CCD video sensor may follow the form dictated by the National Television Standards Committee (NTSC) for video broadcast standards. Such CCD video sensor designs need at least 488 TV lines vertically, 500 to 800 pixels per TV line, have an optical format of 4/3 aspect ratio, and generate field interlaced video at a frame rate of 30 Hz. CCD architectures which achieved the goals of the video format imaging requirements generally fall into two categories: Interline Transfer (ILT) or Frame Transfer (FT) image sensors.
An alternative application of a CCD sensor is industrial inspection or vision equipment. The architecture of a CCD video sensor for this application may be optimized for maximum pixel resolution, or to maximize image frame rate, or both. Often inspection cameras used to inspect moving objects (e.g., on a continuous conveyor belt or rolled goods such as rolls of cloth) employ a line scan CCD sensor where a linear CCD sensor is oriented in a direction perpendicular to the direction of movement of the object being imaged. Advanced linear CCD sensors often employ a time delay and integrate technology and are referred to as TDI CCD sensors.
Two dimensional imaging arrays generally take a snap shot of an image where the whole photoactive array of pixels integrates photogenerated charge for a period of time. At the end of the integration time, the information is transferred from the sensor to an external circuit element using a parallel to serial transfer scheme. Each line of pixel data is transferred into a horizontal readout CCD shift register. The line of data is then transferred serially to an output device at the end of the register.
The data rate at which signal charge can be transferred out of the sensor is limited by a number of factors. Two factors such are the rate at which charge can be transferred along a single CCD readout register and the bandwidth of the output device. Clocking the readout register too fast may result in incomplete charge transfer and resultant image blur. In applications where the sensor data rate must be in excess of the amplifier bandwidth, the horizontal CCD readout shift register is partitioned into subregisters or segments. Each subregister or register segment will then transfer signal charge to its own separate output device which operates at the bandwidth limit of its own output device. The data is then multiplexed off-chip to reconstruct the image at the higher data rate which is the amplifier bandwidth multiplied by the number of output devices or taps. This register architecture is known as a multi-tapped horizontal readout register.
A charge coupled device image sensor employing multiple outputs to increase the total output data transfer rate requires that the CCD register be interrupted in some way at each tap point of its multi-tapped horizontal readout register. This tap may be in the form of a floating diffusion node disposed at multiple points along and within the length of the readout register, or it may be in the form of a floating gate electrode disposed across the horizontal readout register at multiple points along the length of the readout register.
In previous work, multiple "floating diffusion" readout nodes were place in the horizontal readout register such as U.S. Pat. No. 5,608,242 granted to Kamasz. However, such a horizontal readout structure is not bi-directional. To make such a readout register bi-directional, it is necessary to place a diffusion node, and output structure, at each end of each register segment. Such an architecture needs to insert many diffusion nodes within the length of the multi-tapped horizontal readout register.